The goal of this proposal is to elucidate the molecular mechanism(s) by which calmodulin regulates receptor trafficking (transcytosis) across polarized epithelial cells as modeled by FcRn-dependent bi-directional transport of IgG. We have recently observed that calmodulin binds to the FcRn cytoplasmic tail in vitro and that the calmodulin antagonist, W-7, blocks FcRn-dependent apical to basolateral IgG transport across polarized T84 cells. These data suggest that calmodulin may regulate the direction and/or rate of FcRn trafficking. If so, this would be the first cellular protein demonstrated to interact directly with the FcRn tail. Experiments proposed in this application address the hypothesis that calmodulin regulates FcRn trafficking by two mechanisms: by regulating the phosphorylation state of the cytoplasmic tail and/or by mediating interactions between the FcRn cytoplasmic domain and elements of the actin cytoskeleton. We envision that calmodulin functions as a 'molecular switch' redirecting FcRn from its steady-state distribution into a transcytotic pathway of movement across the cell. How calmodulin may orchestrate such complex molecular events is currently unknown. Understanding the process of receptor transcytosis in a polarized cell addresses a central paradigm in cell biology, i.e. how proteins are directed to one surface or the other, and how membrane traffic to the two surfaces is differentially regulated. This project will be co-sponsored by two well-established experts in the field of cell biology: Dr. Wayne I. Lencer (Children's Hospital/Harvard Medical School) and Dr. Paul Matsudaira (Massachusetts Institute of Technology/Whitehead Institute for Biomedical Research). Both the Harvard Medical School and MIT/Whitehead communities provides exceptional resources in cell biology, bioengineering, and protein biochemistry. The three-year KO1 award will provide the applicant with an essential period of advanced training and career development that will significantly enhance her transition to becoming an independent research investigator in the field of gastrointestinal cell biology.